As is well known to those of skill in the art, in an electrophotographic or xerographic reproduction device an electrostatic latent image is formed on a moving photoconductor or photoreceptor that repeatedly cycles through the reproduction process as the photoconductor is reused.
The first process step of such a device can be considered to be the full-surface charging of the photoconductor to a uniform and usually quite high DC voltage, as the photoconductor moves past a charging station such as a charge corona. The charged photoconductor surface is then moved through an imaging station.
In a copier, the imaging station usually comprises an optical system that operates to reflect light off of an original document to be copied. As a result of the reflected light received from the document's white or lightly colored background area, the photoconductor retains a charge only in the area that corresponds to the document's darker or less reflective image area. This latent image is then toned, i.e. covered with toner, as the photoconductor passes through a developing station. Since toner is applied to the charged latent image in a copier, the process is called a charged area development (CAD) process.
In a printer, the imaging station usually comprises a printhead that is driven by binary print data that is supplied by a computer of some type. Laser printheads and LED printheads are two such well known imaging stations. Printers usually operate to discharge the photoconductor in the pattern of the image to be printed, i.e. the printhead usually writes the image to be printed, and as a result the latent image comprises discharged areas of the photoconductor. However, printers can also be configured to write the background, in which case the latent image comprises a charged photoconductor area. In any event, this latent image is then toned, i.e. covered with toner, as the photoconductor passes through a developing station. When toner is applied to the discharged latent image in a printer, the process is called a discharged area development (DAD) process. When toner is applied to the charged latent image in a printer, the process is again called a CAD process.
As will be apparent, the present invention finds utility in either a printer or a copier, and in either a CAD or a DAD process. An embodiment of the invention to be described is that of a DAD printer.
The usual next step of either a copier or a printer process is to transfer a major portion of the toner image that is carried by the photoconductor downstream of the developer station to transfer material, preferably to dielectric transfer material such as paper.
Two types of transfer material may be provided, one being discrete sheets of paper or paper-like material, and the other being a continuous web of paper. The present invention finds utility when individual sheet material is used.
Sheet transfer material is supplied to a transfer station where the paper moves in actual contact, or close proximity to, the photoconductor, so as to in effect cover the photoconductor and its toner image. As one side of the paper is in this close proximity to the photoconductor, the other side of the paper is subjected to the action of a toner transfer station. Two well known transfer stations are roll transfer and corona transfer. In either event, an electrical charge is applied to said other side of the paper, so as to attract toner from the photoconductor to said one side of the paper.
Thereafter, the paper is separated from the photoconductor and is transported to a fusing station whereat the toner is fused to said one side of the paper. The photoconductor is usually discharged and cleaned of residual toner, in preparation for reuse in the reproduction process.
In such a paper sheet device, the individual sheets that are fed to the transfer station are spaced from each other, such that for a period of time no transfer material is intermediate the transfer station and the photoconductor.
The present invention operates to control the transfer station in a manner to produce the same electrical effect on the photoconductor both when a sheet of transfer material resides in the transfer station intermediate the photoconductor and the transfer station, and when no sheet of transfer material is intermediate the photoconductor and the transfer station.
For an unrelated purpose, and for a different end result, U.S. Pat. No. 4,693,593 provides a reproduction device wherein a sensitometric device measures the characteristics of a photoconductor on a test area thereof that intentionally is not an area of the photoconductor that is used for reproduction. In order that this test area be representative of the portion of the photoconductor used for reproduction, steps are taken to ensure that the test area is subjected to the same charge/discharge history as is the portion of the photoconductor that is used in reproduction.
As a feature of the invention the transfer station includes both a transfer corona and a photoconductor erase or quench lamp.
In this regard, U.S. Pat. No. 3,851,230 is of interest in that it discloses transfer means for applying voltage to one side of a transfer-printing sheet, and illumination means for throwing visible light rays onto the photosensitive surface after the transfer-printing sheet has been brought into pressing contact with the photosensitive surface.